In the related art, as a method of sensing an occupant (child) of a child seat installed on a seat of a vehicle, for example, as described in JP 2012-188035A (Reference 1), a method of determining whether or not a child seat belt provided in the child seat is in an installed state, is known. In addition, in Reference 1, a configuration in which it is determined whether or not the occupant who is seated on the child seat is left in the vehicle, and in a case where it is determined that the occupant is in a left state, a warning output is performed, is disclosed.
However, in order to determine whether or not the child seat belt is in an installed state, a condition that a function of detecting the installed state is provided in the child seat in advance becomes a prerequisite. Furthermore, a situation which can be assumed from the fact that the child seat belt is in an installed state, simply, does not exceed a range in which a possibility that the occupant is seated in the child seat is high. Therefore, in the above-described technology in the related art, since there is a problem that a situation in which occupant sensing of the child seat can be used is restricted, regarding this point of view, there is room for improvement.